i got a bold 9900 and running OS 7.1
when i try to make a video clip with HD 1280 * 720 within few second it increase 6mb size
when i to it with normal 640 * 480 it increase more then 6MB file after 13 to 15 seconds
when i use mms mode video size is so small and cant record longer video..
BBM only has a limit of 6MB due to certain reasons only RIM can explain but for my opinion for it would definitely run real slow if it allows us to transfer larger files.
In regard to your video clips, always keep in mind that videos on high resolution eats more memory compared to lower ones simply because it is highly graphical and defined.
MMS videos has its own limit for resolution and length for them to fit in a MMS message.
As mentioned by the prior post, hi def video creates large files. I think the current solution is to upload them to YouTube and then send a link to the video. The upload to youtube should be an option on your phone. I have only done this once and it was best to setup the Youtube account first prior to trying to send the video to YouTube. You can make it a private link so that it doesn't show up with the rest of the Youtube videos.